A variety of home appliances including TVs or refrigerators are operated by receiving commercial alternating current (AC) power from an external source. Electronic devices such as laptop computers, portable telephone terminals, or tablet PCs may also be operated by commercial AC power, or batteries equipped in the electronic devices may be charged by commercial AC power. Such home appliances or electronic devices (hereinafter, collectively referred to as “electronic devices”) include a power device (inverter) that AC/DC converts a commercial AC voltage, or an inverter is equipped in an external power adapter (AC adapter) of an electronic device.
FIG. 1 is a block diagram of an AC/DC converter 400r reviewed by the present inventor. The AC/DC converter 400r mainly includes a rectifying circuit 402, a smoothing capacitor 404, and a DC/DC converter (switching converter) 100r. 
The rectifying circuit 402 is a diode bridge circuit that full-wave rectifies a commercial AC voltage VAC. An output voltage from the rectifying circuit 402 is smoothed by the smoothing capacitor 404 and converted into a DC voltage VDC.
The DC voltage VDC is supplied to an input line 104 of the isolated DC/DC converter 100r at a following stage. The DC/DC converter 100r steps down the DC voltage VDC to generate an output voltage VOUT stabilized to a target value, and supplies the output voltage VOUT to a load (not shown) connected to an output line 106.
The DC/DC converter 100r includes an output circuit 102 and a control circuit 200r. The output circuit 102 includes a switching transistor M1, a detection resistor RCS, a transformer T1, a rectifying diode D1, an output capacitor C1, and a feedback circuit 108. The feedback circuit 108 generates a feedback voltage VFB based on the output voltage VOUT, and supplies the generated feedback voltage V to a feedback (FB) terminal of the control circuit 200r. 
The switching transistor M1 and the detection resistor RCS form a current loop with a primary coil LP of the transformer T1. The rectifying diode D1 and the output capacitor C1 are connected to a secondary coil LS of the transformer T1. A connection point between the switching transistor M1 and the detection resistor RCS is grounded. A voltage drop in proportion to a current IP flowing through the primary coil LP and the switching transistor M1 (hereinafter, referred to as a “current detection signal VIS”) occurs across the detection resistor RCS.
An output terminal OUT of the control circuit 200r is connected to a gate of the switching transistor M1. The control circuit 200r includes a duty controller 202, an overcurrent protection circuit 204, and a driver 206. The duty controller 202 is a modulator of a voltage mode, which generates a pulse signal SPWM having a duty ratio adjusted to allow the output voltage VOUT to be close to a predetermined target value, with reference to the feedback voltage VFB. The driver 206 switches the switching transistor M1 based on the pulse signal SPWM.
The DC/DC converter 100r has an overcurrent protection (OCP) function. That is, when a load current VOUT exceeds a certain threshold value, the DC/DC converter 100r forcibly lowers the output voltage VOUT from a target value VOUT_REF thereof and also reduces the load current IOUT.
The OCP function is mainly realized by resistors R1 and R2 and a voltage detection circuit 114 attached outside of the control circuit 200r and the overcurrent protection circuit 204 contained in the control circuit 200r. 
The voltage detection circuit 114 generates a voltage detection signal VVS based on the output voltage VOUT. The current detection signal VIS generated by the detection resistor RCS and the voltage detection signal VVS generated by the voltage detection circuit 114 are input to a current detection (CS) terminal of the control circuit 200r respectively through the resistors R1 and R2. An electric potential VCS of the CS terminal is a voltage obtained by weight-averaging the current detection signal VIS and the voltage detection signal VVS.
The overcurrent protection circuit 204 compares the detection voltage VCS with a predetermined negative threshold voltage VTH−. When the detection voltage VCS is lower than the threshold voltage VTH−, the overcurrent protection circuit 204 asserts an overcurrent detection signal SOCP (for example, a high level).
When the overcurrent detection signal SOCP is asserted, the duty controller 202 changes the pulse signal SPWM to an OFF level (level corresponding to OFF of the switching transistor M1).
The configuration of the DC/DC converter 100r has been described above. FIGS. 2A and 2B are views illustrating the overcurrent protection in the DC/DC converter 100r of FIG. 1. Specifically, FIG. 2A is a view illustrating voltage-current characteristics of the DC/DC converter 100r, and FIG. 2B is a view illustrating operation waveforms of the DC/DC converter 100r. 
In FIG. 2A, when the load current IOUT is changed within a range from 0 to a certain threshold value IMAX (represented as a region (i)), the output voltage VOUT is maintained as the target value VOUT_REF. In this region, the duty ratio of the pulse signal SPWM is determined based on the input voltage VDC and the target value VOUT_REF of the output voltage VOUT, as illustrated in FIG. 2B.
When the load current IOUT exceeds the threshold value IMAX (region (ii)), voltage feedback is invalidated. As illustrated in FIG. 2B, in this region, a peak value of the coil current IP is limited to a level based on the output voltage VOUT. That is, since a base line VVS of the detection voltage VCS is lowered based on the output voltage VOUT, a variation width of an amplitude component of the detection voltage VCS is reduced. Accordingly, when the peak of the coil current IP is limited, the output voltage VOUT is further lowered, and the peak of the coil current IP is further lowered in a next cycle. The reduction in the coil current IP indicates a reduction in the load current IOUT.
In a region (iii), the output voltage VOUT is substantially lowered down to zero. Then, the coil current IP is substantially stabilized to a predetermined level.
The present inventor has reviewed the DC/DC converter 100r of FIG. 1 and recognized the following problems.
In the DC/DC converter 100r of FIG. 1, the voltage VCS of the CS terminal is changed based on the output voltage VOUT, as well as the coil current IP. Since it assumes that the duty controller 202 is configured as a modulator of a voltage mode, it is difficult to configure the duty controller 202 as a modulator of a current mode.